I got a DDM4V3 upper in 2010. Its bolt carrier is very smooth and stays shiny. It's the old unmarked carrier, the new ones have the double D logo. Anyway, my experience is similar to Purehavoc's. Some finishes are coarser than others and retain oil differently.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek